Some users have complained that their MacBooks have developed a penchant for this problem. In essence, the fault is one in which some - but not all - MacBooks will simply close down during normal use.

This fault appears to affect a small but substantial minority of MacBooks, and usually manifests itself after a period of normal use.

The fault is intermittent and hard to pin down. Apple's technical support department has told some users that re-seating the memory modules may mitigate it in some cases.

The company published a recent technical note advising affected users to contact Apple Support.

Sources have revealed that Apple originally traced the problem to the MacBook logic board.

Early last week, Apple resellers were informed that logic board replacement was no longer necessary in all cases.

Apple's maintenance partners have been instructed to replace the heat sinks in MacBooks first. Logic boards will then be replaced if the problem continues.

Apple last week added heat sinks for MacBooks to its list of available replacement parts for Apple maintenance departments. Initially, only logic boards were available, and the company had advised dealers to replace these if the problem manifested itself. The effect of the demand for logic boards and heat sinks has been to impact on repairs, as spares have been limited.

An Australian report reveals anger among some MacBook owners, who have waited weeks for their new machines to be repaired as a result. An Apple spokesperson told ZDNet: "In terms of the logic board we have now got supply." The company also moved to resolve complaints from some users named in the report.